Multiple electrode vectors for implantable cardiac treatment devices

ABSTRACT

The implantable cardiac treatment system of the present invention is capable of choosing the most appropriate electrode vector to sense within a particular patient. In certain embodiments, the implantable cardiac treatment system determines the most appropriate electrode vector for continuous sensing based on which electrode vector results in the greatest signal amplitude, or some other useful metric such as signal-to-noise ratio (SNR). The electrode vector possessing the highest quality as measured using the metric is then set as the default electrode vector for sensing. Additionally, in certain embodiments of the present invention, a next alternative electrode vector is selected based on being generally orthogonal to the default electrode vector. In yet other embodiments of the present invention, the next alternative electrode vector is selected based on possessing the next highest quality metric after the default electrode vector. In some embodiments, if analysis of the default vector is ambiguous, the next alternative electrode vector is analyzed to reduce ambiguity.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/901,258, filed Jul. 27, 2004; which claims the benefit ofU.S. Provisional Application Ser. No. 60/490,779, filed Jul. 28, 2003,entitled MULTIPLE ELECTRODE VECTORS IN A SUBCUTANEOUS ICD, thedisclosures of which are incorporated herein by reference; thisapplication is also a continuation-in-part of co-pending U.S. patentapplication Ser. No. 10/856,084 filed May 27, 2004, entitled METHOD FORDISCRIMINATING BETWEEN VENTRICULAR AND SUPRAVENTRICULAR ARRHYTHMIAS,which claims the benefit of U.S. Provisional Application Ser. No.60/474,323, filed May 29, 2003; this application is also acontinuation-in-part of co-pending U.S. application Ser. No. 10/863,599,filed Jun. 8, 2004, entitled APPARATUS AND METHOD OF ARRHYTHMIADETECTION IN A SUBCUTANEOUS IMPLANTABLE CARDIOVERTER/DEFIBRILLATOR,which is a continuation of U.S. application Ser. No. 09/990,510, filedNov. 21, 2001, entitled APPARATUS AND METHOD OF ARRHYTHMIA DETECTION INA SUBCUTANEOUS IMPLANTABLE CARDIOVERTER/DEFIBRILLATOR, now U.S. Pat. No.6,754,528; this application is also a continuation-in-part of U.S.patent application Ser. No. 10/858,598 filed Jun. 1, 2004, entitledMETHOD AND DEVICES FOR PERFORMING CARDIAC WAVEFORM APPRAISAL, whichclaims the benefit of U.S. Provisional Application Ser. No. 60/475,279,filed Jun. 2, 2003, and the disclosure of each of these applications isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to methods and devices forimproving sensing in an implantable cardiac treatment system. Moreparticularly, the present invention relates to the placement ofelectrodes in an implantable pacing or cardioversion/defibrillationsystem at defined locations within a patient to create multipleelectrode vectors for improved far-field sensing and improved sensing ofcardiac events.

BACKGROUND

Implantable cardiac rhythm management devices are an effective treatmentin managing irregular cardiac rhythms in particular patients.Implantable cardiac rhythm management devices are capable of recognizingand treating arrhythmias with a variety of therapies. These therapiesinclude anti-bradycardia pacing for treating bradycardia,anti-tachycardia pacing or cardioversion pulsing for treatingventricular tachycardia, and high energy shocking for treatingventricular fibrillation. Usually, the cardiac rhythm management devicedelivers these therapies for the treatment of tachycardia in sequencestarting with anti-tachycardia pacing and then proceeding to low energyshocks, and then finally to high energy shocks. Sometimes, however, onlyone of these therapies is selected depending upon the tachyarrhythmiadetected.

To effectively deliver treatment, cardiac rhythm management devices mustfirst accurately detect and classify a cardiac event. Through theaccurate classification of cardiac events, these cardiac rhythmmanagement devices are able to classify the type of arrhythmia that isoccurring (if any) and assess the appropriate therapy to provide to theheart (if indicated). A problem arises, however, when the cardiac rhythmmanagement device misclassifies an event and, as a result, deliversinappropriate therapy or fails to deliver therapy.

Besides being physically painful to the patient, when a cardiac rhythmmanagement device delivers inappropriate treatment, it can be extremelydisconcerting. Moreover, delivery of an inappropriate therapy canintensify the malignancy of the cardiac arrhythmia or cause anarrhythmia where one was not present. The accuracy of a sensingarchitecture is, therefore, an important factor in ensuring thatappropriate therapy is delivered to a patient.

SUMMARY

In a first embodiment, an implantable cardiac treatment system isprovided with electrodes disposed at several locations in a patient'sthorax. During operation of the system, various sensing vectors can beperiodically, repeatedly, or continuously monitored to select the bestsensing vector for event detection and classification. A sensing vectormay be selected and then used for analysis. In another embodiment,multiple vectors may be simultaneously analyzed to provide a tiered orprioritized detection scheme, or to provide a secondary check on ahigher priority vector. For example, a first vector may be used as thehigher priority vector, and a second vector may be used to verify thatsensed with the first vector. Alternatively, ambiguity may be reduced bythe use of a second vector to check on a first vector. Additionalembodiments include implantable cardiac treatment systems andoperational circuitry for use in implantable cardiac treatment systemswhich are adapted for performing these methods. Some embodiments takethe form of subcutaneous implantable cardiac treatment systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B illustrate, respectively, representative subcutaneous andintravenous implantable cardiac treatment systems;

FIG. 2 shows a subcutaneous implantable cardiac treatment system havingan alternative subcutaneous electrode system arrangement;

FIGS. 3A and 3B show three positions for the placement of an implantablecardiac treatment device and four subcutaneous positions for theplacement of an electrode;

FIG. 4 illustrates a laterally placed implantable cardiac treatmentsystem with a parasternally placed electrode;

FIG. 5 illustrates a pectorally placed implantable cardiac treatmentsystem with a parasternally placed electrode;

FIGS. 6A-6F depict recorded electrocardiograms from several discreteintra-electrode distances;

FIG. 7 shows a block diagram of the vector sensing evaluation fordetermining the periodicity to evaluate the best electrode vector basedon observed ambiguous signals; and

FIGS. 8A and 8B show the relationships between two electrode vectors onsensing a cardiac depolarization vector.

DETAILED DESCRIPTION

The following detailed description should be read with reference to theFigures, in which like elements in different Figures are numberedidentically. The Figures, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope of theinvention. Those skilled in the art will recognize that many of theexamples and elements of the examples have suitable alternatives thatmay be utilized.

The present invention is generally related to cardiac rhythm managementdevices (e.g., an Implantable Cardioverter/Defibrillator (ICD) system)that provide therapy for patients experiencing particular arrhythmias.The present invention is directed toward sensing architectures for usein cardiac rhythm management devices. In particular, the presentinvention is suited for ICD systems capable of detecting anddefibrillating harmful arrhythmias. Although the sensing architecture isintended primarily for use in an implantable medical device thatprovides defibrillation therapy, the invention is also applicable tocardiac rhythm management devices directed toward anti-tachyarrhythmiapacing (ATP) therapy, pacing, and other cardiac rhythm devices capableof performing a combination of therapies to treat rhythm disorders,including external devices.

To date, ICD systems have been epicardial systems or transvenous systemsimplanted generally as shown in FIG. 1B, however, as further explainedherein, the present invention is also adapted to function with asubcutaneous ICD system as shown in FIG. 1A.

FIG. 1A illustrates a subcutaneously placed ICD system. In thisillustrative embodiment, the heart 1 is monitored using a canister 2coupled to a lead system 3. The canister 2 may include an electrode 4thereon, while the lead system 3 connects to sensing electrodes 5, 6,and a coil electrode 7 that may serve as a shock or stimulus deliveryelectrode as well as a sensing electrode. The general path betweenvarious electrodes defines a number of sensing vectors V1, V2, V3, V4.It can be seen that each vector provides a different vector “view” ofelectrical activity in the heart 1. The system may be implantedsubcutaneously as illustrated, for example, in U.S. Pat. Nos. 6,647,292and 6,721,597, the disclosures of which are both incorporated herein byreference. By subcutaneous placement, it is meant that sensing andtherapy can be accomplished with electrode placement that does notrequire insertion of an electrode into a heart chamber, the heartmuscle, or the patient's vasculature.

FIG. 1B illustrates a transvenous ICD system. The heart 10 is monitoredand treated by a system including a canister 11 coupled to a lead system12 including atrial electrodes 13 and ventricular electrodes 14. Anumber of configurations for the electrodes may be used, includingplacement within the heart, adherence to the heart, or dispositionwithin the patient's vasculature. For example, Olson et al., in U.S.Pat. No. 6,731,978, illustrate electrodes disposed in each chamber ofthe heart for sensing, as well as shocking electrodes in addition to thesensing electrodes.

The present invention, in some embodiments, is also embodied byoperational circuitry including select electrical components providedwithin the canister 2 (FIG. 1A) or canister 11 (FIG. 1B). In suchembodiments, the operational circuitry may be configured to enable themethods to be performed. In some similar embodiments, the presentinvention may be embodied in readable instruction sets such as a programencoded in machine or controller readable media, wherein the readableinstruction sets are provided to enable the operational circuitry toperform the analysis discussed herein in association with variousembodiments. Further embodiments may include a controller ormicrocontroller adapted to read and execute embodiments discussedherein.

In the system illustrated in FIG. 1A, the subcutaneous implantablecardiac treatment device can sense a plurality of electrode vectors. Inparticular, the configuration depicted can sense at least between thefirst sensing electrode 6 and the canister or housing electrode 4. Thecanister or housing electrode 4 can be a part of the housing orcanister, the housing or canister itself may be an electrode 4, oralternatively, the electrode can be attached to or on the housing. Thissensing relationship forms electrode vector v₁. The device can furthersense between the first sensing electrode 6 and the second sensingelectrode 5 to form electrode vector v₂. A third sensing configurationis created by sensing between the second sensing electrode 5 and thecanister electrode 4. This sensing relationship forms electrode vectorv₃. The last illustrated electrode vector is between the shockingelectrode 7 and the canister electrode 4 forming electrode vector v₄.The system depicted in FIG. 1 a is illustrative only. The purpose of theFigure is to demonstrate some of the possible electrode vectors that canbe formed with implantable cardioverter-defibrillator systems,particularly with subcutaneous systems. Other electrode arrangements andelectrode types may be utilized without deviating from the spirit andscope of the invention.

An alternative subcutaneous embodiment is depicted in FIG. 2. A canister18 is electrically coupled to electrodes 19, 20, 22, with electrodes 19,20 disposed on a lead 24 and electrode 22 disposed on the canister 18.The several electrodes 19, 20, 22 provide various sensing vectors aroundheart 26. The illustrative leads and electrodes may have variouslengths. As further discussed below, certain sizes and lengths mayprovide advantageous sensing characteristics.

FIGS. 3A and 3B show three illustrative subcutaneous positions (X, Y andZ) for the placement of an ICD in a patient's thoracic region. FIG. 3Ais a view from the front, facing a patient's chest, while FIG. 3B is aview from the left of a patient, each view showing only the ICDcomponents and the heart. Position X is disposed on the left side of therib cage, inferior to the arm, and is designated herein as the lateralposition. Position Y is a frontal position, inferior to the inframammarycrease (IC) and is designated herein as the inframammary position.Finally, position Z is also a frontal position and can correspond to aconventional positioning for ICDs. This position is located superior andto the left of the heart (H) and inferior the collarbone (CB). Thisposition Z is designated herein as the pectoral position.

Similarly, FIGS. 3A and 3B show four subcutaneous positions (A, B, C andD) for the placement of the subcutaneous electrode system 12 upon apatient's thoracic region. Position A is a parasternal placement that ispositioned on the left side of the sternum (ST). Position B is anelectrode placement that runs parallel to the sternum (ST), but positionB is located laterally as opposed to the parasternal placement ofposition A. Position C is an electrode placement that is generallyorthogonal to positions A and B and is positioned on a line superior tothe heart (H). Finally, position D is an electrode placement that isparallel with position C, but has the electrode positioned in a lineinferior to the patient's heart (H).

FIG. 4 illustrates a laterally placed (X) ICD canister 30 with aparasternally placed (position A) subcutaneous electrode system alonglead 32. FIG. 4 shows the lead 32 traversing subcutaneously along theribcage and terminating in a position where the subcutaneous electrodesystem of the lead 32 is disposed vertically and parallel to thepatient's sternum (ST). The first sensing electrode 34 is shownpositioned at or near a line superior to the patient's heart (H). A coilelectrode 35 is also shown, with the coil electrode 35 coupled for useas a shocking electrode, and, optionally, as an additional sensingelectrode.

FIG. 5 similarly illustrates a pectorally placed (Z) ICD canister 36with a parasternally placed (position A) subcutaneous electrode systemincluding a lead 38. FIG. 5 also shows the lead 38 traversingsubcutaneously along the ribcage and terminating such that thesubcutaneous electrode system of the lead 38 is disposed vertically andparallel to the patient's sternum (ST). In contrast to the electrodeplacement in FIG. 3, the first sensing electrode 40 of the subcutaneouselectrode system is positioned at or near a line inferior to thepatient's heart (H). Again, a coil electrode 41 serving as a shockingand, if desired, sensing electrode is also illustrated.

The subcutaneous space surrounding a patient's thoracic region isinherently curvaceous. Because the canister 30, 36 (which may include asensing electrode) and the subcutaneous electrode system on leads 32, 38are positioned upon this region, the electrodes, canister and lead forthe ICD are rarely, if ever, planar with respect to one another. Thusvarious vectors may be defined to intersect the heart (H), withoutnecessarily having to place electrodes in the heart (H).

The distance separating the canister 30, 36 and the electrodes on theleads 32, 38 is dependent on the patient's anatomy. With theconfigurations shown in FIGS. 4 and 5, in a typical adult patient, thecenter of the canister 30, 36 is approximately 8 cm to approximately 19cm away from the center of a shocking coil 35, 41 on the leads 32, 38.Children receiving devices according to the present invention may haveseparations between the canister and the shocking coil 35, 41 ofgenerally no less than approximately 4 cm.

Subcutaneous embodiments of the present invention benefit from theability to optimize the intra-electrode distance to maximize the sensingof cardiac electrical activity. Because subcutaneous embodiments of thepresent invention are not constrained by the location of electrodeswithin the system or within the patient's thorax, a subcutaneous systemmay use intra-electrode distances particularly chosen for optimizingfar-field signal sensing, or may vary the sensing electrode pair duringoperation to optimize sensing.

FIG. 6A-6F depict observed electrocardiogram (EKG) signals from twosmall surface area electrodes having differing intra-electrodedistances. In these figures, one of the two small surface areaelectrodes was placed in a fixed position located laterally 0.5″ fromthe sternum, and over the patient's heart. The second of the two smallsurface area electrodes was positioned specific distances from the firstelectrode to observe and record the change in the resulting EKG.

Initially, the second electrode was placed laterally 0.75″ from thefixed electrode, thereby creating an intra-electrode distance ofapproximately 0.75″. An EKG was then observed of the cardiac electricalactivity. FIG. 6A represents a portion of the recorded EKG where theelectrodes possessed an intra-electrode distance of approximately 0.75″.Additional EKGs were recorded to measure the sensed cardiac activityafter positioning the second electrode laterally approximately 1.25″,2″, 2.5″, 3.25″ and 5.5″ away from the fixed electrode position. Theresulting EKGs are shown in FIGS. 6B-6F, respectively. The averageobserved amplitude for the QRS complex was approximately 1.0 mV in FIG.6A, approximately 2.0 mV in FIG. 6B, approximately 4.4 mV for FIG. 6C,approximately 5.5 mV for FIG. 6D, approximately 7.8 mV for FIG. 6E andapproximately 9.6 mV for FIG. 6F.

Subcutaneous embodiments of the present invention are not constrained bythe location of electrodes to intravenous or intracardiac locations. Assuch, the subcutaneous system may use intra-electrode distances that areparticularly chosen for optimizing far-field signal sensing. It isobserved in FIGS. 6A-6F that increasing the intra-electrode distanceresults in significantly increased signal amplitudes. A 100% increase inamplitude was observed between the recorded cardiac electrical activityin FIG. 6B and FIG. 6A. A 340% increase in amplitude was observedbetween the recorded cardiac electrical activity in FIG. 6C and FIG. 6A.A 450% increase in amplitude was observed between the recorded cardiacelectrical activity in FIG. 6D and FIG. 6A. A 680% increase in amplitudewas observed between the recorded cardiac electrical activity in FIG. 6Eand FIG. 6A. Finally, an 860% increase in amplitude was observed betweenthe recorded cardiac electrical activity in FIG. 6F and FIG. 6A.

It is appreciated by those skilled in the art that it is desirable toobtain the highest signal amplitudes possible when sensing.Specifically, because detected cardiac electrical signals are processedto classify particular rhythms, the larger the cardiac electrical signalthe greater the opportunity to correctly classify a rhythm. Someembodiments of the present invention provide an enhanced opportunity tocorrectly classify arrhythmias by using intra-electrode distancesparticularly chosen for optimizing far-field signal sensing.

Some embodiments of the present invention are further capable ofchoosing the most appropriate electrode vector to sense within aparticular patient. In one embodiment, (referring to FIG. 1) afterimplantation, the ICD is programmed to sense between several availableelectrode vectors—v₁, v₂, v₃ and v₄. The ICD system then senses a seriesof cardiac signals using some or all of the available electrode vectors,or a preset number of available electrode vectors. In certainembodiments, the ICD system then determines the most appropriateelectrode vector for continuous sensing based on which electrode vectorresults in the greatest signal amplitude, or performs best using someother metric such as signal-to-noise ratio (SNR). The electrode vectorpossessing the highest quality metric (e.g., amplitude or SNR) is thenset as the default electrode vector for continuous sensing. In certainembodiments, the next alternative electrode vector is selected based onbeing generally orthogonal to the default electrode vector. For example,if electrode vector v₃, is selected as the default vector, the nextalternative electrode vector may be v₂, an electrode vector generallyorthogonal to v₃. In yet other embodiments the next alternativeelectrode vector is selected based on possessing the next highestquality metric after the default electrode vector.

Recognizing that patient anatomies vary, the present invention is notintended to be limited to purely or strictly orthogonal sensing vectors.In some embodiments, generally orthogonal sensing vectors are consideredto exist when two sensing vectors create an angle such that themagnitude of the cosine of the angle is less than about 0.7. In anotherembodiment, the magnitude of the cosine of the angle is less than about0.5. In a further embodiment, the magnitude of the cosine of the angleis less than about 0.3. As used herein, the phrase “the magnitude of”indicates absolute value when applied to a scalar value such as thecosine of an angle. This angular analysis is used herein because, whiletwo vectors may define a plane, an intersection of two vectors candefine a plurality of angles. Analysis in terms of cosines assures thesame result regardless how the vectors are disposed with respect to oneanother for the purpose of determining the angles therebetween. Dealingonly in first quadrant angles, the above noted values for cosines yieldangles of between about 45 and 90 degrees, about 60 and 90 degrees, andabout 72 and 90 degrees.

In one embodiment of the present invention, the ICD system determinesthe most appropriate electrode vector based on results of an operationperformed on all of the sensed signals. The ICD system independentlyoperates on all of the sensed signals received from each of the possibleelectrode vectors using the ICD system's detection architecture. Forexample, the ICD system may run all of the signals from each of theelectrode vectors through a correlation waveform analysis, or a similaroperation function. Specifically, the ICD system performs a correlationwaveform analysis on electrode vectors v₁, v₂, v₃ and v₄ independently.The ICD system then evaluates the results from each of the independentlyoperated-on signals. This evaluation procedure determines the electrodevectors that yield the highest quality metric for rendering a decision.Finally, the ICD system selects the electrode vector yielding thehighest quality metric as the default electrode vector for continuoussensing. For example, the ICD system will select the electrode vector v₃as the default electrode vector if it yields the highest quality metricfrom the four electrode vectors evaluated.

In certain embodiments, the ICD system paretos (prioritizing accordingto the hierarchy of performance) the electrode vectors. By paretoing theelectrode vectors, the ICD system may utilize alternative electrodevectors, in particular the next best performing electrode vectors, whenambiguities arise in analysis of the default electrode vector.

For certain embodiments of the present invention, the evaluation of thebest electrode vectors for sensing are updated periodically by thephysician. A programmer responsive to the ICD system may receivetransmissions from the ICD system. Amongst others, the transmissionsfrom the programmer characterize the cardiac activity sensed by eachelectrode vector. The physician may then select the optimal electrodevector for the particular patient and set that chosen electrode vectoras the default. The programmer may additionally enable the physician toelect alternative schemes for instances where the signal from thedefault electrode vector is compromised. Additionally, the programmermay select the optimal electrode vector and elect alternative schemesautomatically based on the received transmissions from the ICD system.

In yet alternative embodiments, the evaluation of the best electrodevectors for sensing is updated periodically by the ICD system, whetherthat decision is made a priori (e.g., by signal amplitude) or ex postfacto (e.g., after operating on the unprocessed signal data). Forexample, initially the highest quality metric (e.g., highest amplitudesignal) is sensed using electrode vector v₁. Sometime afterimplantation, however, the ICD system may determine that the highestquality metric is experienced when sensing through the electrode vectorv₂. Conversely, it may be periodically determined that the bestelectrode vector continues to remain with electrode vector v₁ during theentire life of the device.

An example of an a priori update would be one where the SNR is measuredfor each of several vectors over time. If a muscle artifact developsafter implantation, or if a fibroid forms around one of the sensingelectrodes, then the relative SNR of the several sensing vectors maychange over time. If one of the sensing vectors provides a superior SNRto that of the initially chosen vector, then the later update may selecta different vector.

An example of an ex post facto update would be one where a particularsensing vector is chosen for a period of time, but proves to beunsuitable for analysis, for example, due to noise artifacts. Forexample, if a beat validation scheme is used as explained in co-pendingU.S. patent application Ser. No. 10/858,598 filed Jun. 1, 2004, entitledMETHOD AND DEVICES FOR PERFORMING CARDIAC WAVEFORM APPRAISAL, thedisclosure of which is incorporated herein by reference, then consistentfailure to capture validated beats may indicate that the chosen vectoris unsuitable. Likewise, if a template formation system relies uponcaptured data, then a failure to capture a template meeting chosenvalidity criteria may indicate that the chosen vector is unsuitable. Insuch cases, another sensing vector may be chosen by looking at the nextbest sensing vector. For example, if a first vector is chosen forsensing because it has a best amplitude of sensed vectors, supposingthat first vector proves to be unsuitable for template formation, then asecond vector having the second best amplitude may be chosen.

The periodicity used to evaluate the best electrode vector is preferablybased on whether the sensed cardiac electrical signal is ambiguous tothe ICD system's detection architecture. With respect to this invention,ambiguity concerns whether the sensed cardiac electrical signal isdifficult to comprehend, understand, or classify by the ICD system'sdetection architecture. This process is illustrated by example in FIG.7.

Referring now to FIG. 7, a cardiac electrical signal is sensed throughelectrode vector v₁. The sensed signal is then operated on by thedetection architecture of the ICD system. The result of this operationis then evaluated. In certain embodiments, the ICD system will evaluatewhether the operated-on signal equates unambiguously to a normal sinusrhythm. If the result of the operation unambiguously indicates a normalsinus rhythm, then the ICD system repeats the procedure and sensesanother cardiac electrical signal to operate upon. However, if theresult of the operation is ambiguous, or the operated-on signalindicates a rhythm other than normal sinus, then the process enters asecond stage 50. Some illustrative explanations of ambiguity can befound in U.S. patent application Ser. No. 10/856,084 filed May 27, 2004,entitled METHOD FOR DISCRIMINATING BETWEEN VENTRICULAR ANDSUPRAVENTRICULAR ARRHYTHMIAS, the disclosure of which is incorporatedherein by reference.

In the second stage 50, the sensing of the next cardiac electricalsignal in time is performed through an alternative electrode vector. Insome embodiments, the alternative electrode vector used for this sensingis one that is generally orthogonal to the electrode vector used tosense the previous signal. For example, if the previous cardiacelectrical signal was sensed through electrode vector v₁, the nextcardiac electrical signal would be sensed through electrode vector v₂.In alternative embodiments of the present invention, any of theremaining electrode vectors may be used to sense the next cardiacelectrical signal in the second stage 50. For example, a next highestamplitude sensing vector may be chosen.

This subsequently sensed signal is then operated on by the detectionarchitecture of the ICD system. The result of this operation is againevaluated. If the result of the operation unambiguously indicates anormal sinus rhythm from this alternative electrode vector, then the ICDsystem repeats the procedure and senses another cardiac signal tooperate upon. In certain embodiments, subsequently sensed cardiacsignals following the second stage 50 continue to be sensed through theelectrode vector used for evaluation in the second stage 50. Thus in theprevious example, all subsequently sensed cardiac electrical signalswould be sensed using electrode vector v₂. However, in particularembodiments, this is only true if the result of the second stage 50operation unambiguously indicates a normal sinus rhythm. If the resultof the second stage 50 is again ambiguous, or the operated-on signalunambiguously indicates a rhythm other than normal sinus, then futuresensed cardiac electrical signals may once again be processed using thedefault electrode vector—here being v₁.

In yet alternative embodiments, the next cardiac electrical signalfollowing any second stage 50 evaluation is again initially sensedthrough the default electrode vector—for this example v₁. In thisembodiment, the default electrode vector is changed only after a seriesof unambiguous evaluations utilizing the second stage 50 and itsalternative electrode vector.

The ICD device of the present invention may also sense between multipleelectrode vectors continuously and/or independently of one another. Thisability allows the present invention to evaluate the same cardiacelectrical signal in time from numerous vector viewpoints. Additionally,this ability permits the ICD system to evaluate the best electrodevector based on observed ambiguous signals without failing to operateand evaluate each sensed cardiac signal. Specifically, a cardiacelectrical signal is sensed through an electrode vector, for example,v₁. The sensed signal is then operated on by the detection architectureof the ICD system. The result of this operation is then evaluated. Ifthe result of the operation is ambiguous, or the operated-on signalunambiguously indicates a rhythm other than normal sinus, then theprocess enters a second stage 50.

In the second stage 50 of this embodiment, a cardiac electrical signalsensed at the same time as the sample already evaluated, but withdifferent electrodes, is evaluated. Therefore, both the signalpreviously operated on and the one which is to be operated on in thesecond stage 50 occurred at the same time—although acquired through adifferent electrode vector. The sensed signal from v₂ is then operatedon by the detection architecture of the ICD system. The result of thisoperation is again evaluated. If the result of the operationunambiguously indicates a normal sinus rhythm in this second electrodevector, then the ICD system repeats the procedure and senses anothercardiac electrical signal in which to operate upon.

The general ability to sense between multiple sensing vectorsparticularly enhances specificity for detection architectures thatdiscriminate between arrhythmias. Specifically, sensing between multipleelectrode vectors enhances specificity in discriminating the origin andtype of arrhythmia. In one example of the present invention, a cardiaccomplex representative of normal sinus rhythm (NSR) is captured fromeach of electrode vector v₁ and electrode vector v₂, and then stored.These are stored as NSR template 1 and NSR template 2, respectively.Because electrode vectors v₁ and v₂ are at different angles to theheart, their respective templates may differ significantly even thoughthey may be based upon the same cardiac events.

From beat to beat, sensed complexes may be compared to their respectiveNSR templates. As an example, in certain vector orientations ventricularoriginating arrhythmias may resemble an NSR. With ICD systems that senseonly one electrode vector, some ventricular arrhythmias may not bedistinguishable to a detection architecture. In the present invention,however, the chances of failing to classify a particular rhythm arereduced through the use of multiple views. In particular, although aventricular originating arrhythmia may resemble the NSR template in oneview, it would be highly unlikely that a second electrode vector wouldalso sense the same complex as resembling its NSR template.

Ventricular originating arrhythmias often exhibit a polarity flip withrelation to their NSR. If this polarity flip goes undetected because ofpositioning in one electrode vector, a generally orthogonally positionedsecond electrode vector would most likely sense such a flip whencompared to its NSR template. Thus, the detection algorithm wouldclassify the uncharacteristic complex, or series of complexes, andassess the complexes as a ventricular arrhythmia.

In one embodiment, an initial analysis of the default electrode vectorcaptured using a default electrode pair may yield an ambiguous result.For example, if a correlation waveform analysis is performed to comparea sensed signal to an NSR template, the waveform analysis may indicatethat NSR is not occurring. However, it may not be clear from the initialanalysis what type of arrhythmia is occurring (for example, asupraventricular arrhythmia which does not require treatment, or aventricular arrhythmia that does require treatment). In the illustrativeexample, a second level of analysis may be performed using a signalcaptured using different electrodes to differentiate treatable anduntreatable arrhythmias. The method may then return to observing onlythe default electrode pair.

FIGS. 8A and 8B demonstrate the relationship between two electrodevectors in sensing a cardiac depolarization vector. More specifically,FIGS. 8A and 8B graphically illustrate the electrode vectors formed inthe ICD system between the active canister 64 and the first sensing ring62, and the first sensing ring 62 and the second sensing ring 60. Thesevectors are labeled, respectively, v₁ and v₂. FIGS. 8A and 8B furtherillustrate a cardiac depolarization vector M. The cardiac depolarizationvector M cannot be completely described by measuring only one of the twoelectrode vectors shown in FIGS. 8A and 8B. More information about thecardiac depolarization vector M can be acquired using two electrodevectors. Thus, the resulting ECG derived from three or more electrodeswill more accurately define a depolarization vector M, or a fractionthereof.

For the cardiac depolarization vector M, the voltage induced in thedirection of electrode vector v₁ is given by the component of M in thedirection of v₁. In vector algebra, this can be denoted by the dotproductυ _(v1) =M·v ₁where υ_(v1) is the scalar voltage measured in the direction ofelectrode vector v₁. FIGS. 8A and 8B further depict an electrode vectorv₂ oriented in space. The effect of the cardiac depolarization vector Mas it relates to electrode vector v₂ differs, however, between FIGS. 8Aand 8B.

FIG. 8A illustrates a cardiac depolarization vector M that includescomponents in both vector directions, and so is sensed and measured withscalar voltages along both electrode vectors. The cardiac depolarizationvector M in FIG. 8A is oriented in space such that both electrodevectors v₁ and v₂ sense scalar voltages υv₁ and υ_(v2), respectively.Although the scalar voltage υ_(v1) predominates, the scalar voltageυ_(v2) is sensed and can be used for discriminating differences in themagnitude and the direction of the cardiac depolarization vector M.

In contrast, the electrode vector v₂ in FIG. 8B is oriented orthogonallyto the cardiac depolarization vector M. In this embodiment, thecomponent of M along the direction of vector electrode v₂ is zerobecause the v₂ electrode vector senses no voltage as a result of thecardiac depolarization vector; no voltage is induced in the direction ofv₂. In contrast, the scalar voltage along v₁ parallels thedepolarization vector M and fully captures M.

With the ability to ascertain the cardiac depolarization vector M, FIGS.8A and 8B further depict how the present invention may be utilized toenhance a particular attribute of the sensed signal. For example, thepresent invention may be utilized to enhance the signal-to-noise ratio(SNR) for an ICD system. In illustration, suppose that most patientsdemonstrate a cardiac depolarization vector M similar to that depictedin FIG. 8A. For these patients, sensing along electrode vector v₁ alonewould result in a sufficiently high SNR to sense and detect mostarrhythmias, while vector v₂ provides information that may be relevantfor sensing if analysis of v₁ contains some ambiguity.

There may be patients, however, who exhibit a cardiac depolarizationvector M similar to the one depicted in FIG. 8B. These patients couldexhibit a cardiac depolarization vector M at the time of implant, orafter developing a pathology that changes the cardiac depolarizationvector M over time to represent the one depicted in FIG. 8B. For thesepatients, sensing along electrode vector v₂ alone would result in anextremely low SNR. Furthermore, the ICD system may not be able to detectcertain arrhythmic events if this were the only sensing vector the ICDsystem possessed. However, knowledge that v₂ has such a low magnitudeindicates greater directional information than just analyzing v₁.

As described above, sensing sensitivity depends on the orientation ofthe cardiac depolarization vector M with respect to the orientation ofthe sensing electrodes.

As noted above, correlation techniques involving template comparisonsmay be used in the signal analysis of some embodiments of the presentinvention. Such analysis may be used to discriminate certain arrhythmiasand direct appropriate therapy. For example, arrhythmias that indicatetreatment include monomorphic ventricular tachycardia (MVT), polymorphicventricular tachycardia (PVT) and ventricular fibrillation (VF). Theseare arrhythmias that are considered malignant and therefore requiretherapy from an implantable device such as an ICD. In contrast, certainarrhythmias do not indicate treatment. For example, somesupraventricular arrhythmias that may not indicate treatment includeatrial fibrillation (AF), atrial tachycardia (AT) and sinus tachycardia(ST).

Referring now to Table 1, a comparison chart is depicted representingseveral comparison methods (outlined in detail below) and the predictedoutcomes of these comparisons with various arrhythmias. Several of themethods make use of correlation waveform analysis (CWA). The arrhythmiasin Table 1 include both ventricular arrhythmias requiring therapy andsupraventricular arrhythmias where therapy should be withheld. AlthoughTable 1 describes several comparison methods to aid in discriminatingbetween arrhythmias, the present invention is not limited in terms ofthe scope of Table 1. Other comparison methods may be used, and arecontemplated, to populate a similar table for discriminating betweenarrhythmias. TABLE 1 AF AT/ST MVT PVT VF A HIGH HIGH LOW LOW LOW B LOWLOW LOW HIGH HIGH C HIGH HIGH HIGH LOW LOW D LOW LOW LOW HIGH HIGH ENARROW NARROW WIDE WIDE WIDE F LOW HIGH HIGH LOW LOW G NO YES/NO YES YESYESTable 1 uses the following comparison methods and their correspondingdefinitions:

-   -   A=CWA between a sensed complex and a stored sinus template,        where HIGH indicates high correlation with a stored sinus        template and LOW indicates low correlation with a stored sinus        template;    -   B=Variability in the CWA between a sensed complex and a stored        sinus template, where HIGH indicates high variability within a        grouping of cardiac complexes and LOW indicates low variability        within a grouping of cardiac complexes;    -   C=CWA between a sensed complex and a template acquired after a        triggering event (here, a template representative of a complex        with a rate between 170 and 260 bpm), where HIGH indicates high        correlation with the template acquired after a triggering event        and LOW indicates low correlation with the template acquired        after a triggering event;    -   D=Variability in the CWA between a sensed complex and a template        acquired after a triggering event (here, a template        representative of a complex with a rate between 170 and 260        bpm), wherein the template is dynamic and continually updated by        the previously sensed cardiac complex, where HIGH indicates high        variability in the CWA, within a grouping of cardiac complexes        when compared to a template acquired after a triggering event        and LOW indicates low variability in the CWA within a grouping        of cardiac complexes when compared to a template acquired after        a triggering event;    -   E=Comparison to a QRS width threshold value (described in detail        above), where WIDE indicates QRS waveforms having greater that        35 percent of their complex laying above the width threshold        value and NARROW indicates QRS waveforms having less that 35        percent of their complex laying above the width threshold value;    -   F=Interval rate stability of +/−30 milliseconds, where YES        indicates stability within +/−30 milliseconds and NO indicates        stability outside of +/−30 milliseconds; and    -   G=A rate acceleration event, where YES indicates a rate        accelerating event and NO indicates the lack of a rate        accelerating event.

Thresholds indicating whether a CWA score is “high” or “low” may be setin any suitable manner. Some examples of such analysis are shown inco-pending U.S. patent application Ser. No. 10/856,084 filed May 27,2004, entitled METHOD FOR DISCRIMINATING BETWEEN VENTRICULAR ANDSUPRAVENTRICULAR ARRHYTHMIAS.

Certain arrhythmias in Table 1 are strongly indicated when processedthrough certain comparison methods. These results are unambiguous evenwhen sensed by transvenous lead systems. An example of this phenomenonis the strong indications observed in PVT and VF arrhythmias whenrunning comparison method A. Specifically, a sensed PVT or VF arrhythmiccomplex will almost always correlate poorly (score as LOW) when comparedto a stored sinus template. The ambiguity in this comparison isextremely low with these arrhythmias. Thus, scoring a LOW in comparisonmethod A lends itself to a strong indication for these two particulararrhythmias.

Table 2 shows which of the illustrative comparison methods tease outparticular arrhythmias with little to no ambiguity, or alternatively,show a strong indication for the particular arrhythmia. TABLE 2 AF AT/STMVT PVT VF A — — — LOW LOW B — LOW LOW HIGH HIGH C — — — LOW LOW D — LOWLOW HIGH HIGH E — — — — — F — — — LOW LOW G — —/NO — — —

Certain entries in Table 1 are influenced by some ambiguity. BecauseTable 1 was tabulated from data observed by transvenous lead systems,these systems cannot always unambiguously discern vector informationthat distinguishes attributes specific to particular arrhythmias. Thereason for this is that transvenous electrode systems are optimized forlocal information sensing, and this optimization comes at the expense offar field and vector information sensing. This relative lack of farfield and vector information sensing translates to relatively frequentambiguous sensing with certain arrhythmias, such as an atrialfibrillation that conducts rapidly to the ventricles.

The ambiguity of certain arrhythmias can be high in particularcomparison methods. Table 3 shows which of the illustrative comparisonmethods tease out particular arrhythmias with high ambiguity and theircorresponding estimate of ambiguity percentage for a transvenousapproach. Table 3 shows the weak indicators for particular arrhythmias.Again, this ambiguity is primarily the result of the data propagatingTables 1-3 being observed from transvenous lead systems. TABLE 3 AFAT/ST MVT PVT VF A — — LOW — — (20%) B — — — — — C — — HIGH — — (20%) D— — — — — E NARROW NARROW — — — (33%) (33%) F — — — — — G NO (20%) — —YES YES (20%) (20%)

Of note, the ambiguity percentages used in Table 3, and all subsequentTables, are educated estimations based on published studies and clinicalobservations. It is believed that these results are suitable forextrapolation to a larger population. However, ambiguities in the Tablesexist. For example, it is estimated that about 20% of the populationwill contraindicate an MVT when using either comparison method A orcomparison method C. For some embodiments of the present invention,these ambiguity percentages provide a tool for planning amulti-comparison methodology. By knowing the relative ambiguities of anyparticular comparison method, the detection enhancement operator maydetermine particular comparison methods more efficaciously over otherswhen discerning particular arrhythmias.

An example of the ambiguity of certain arrhythmias when using certaincomparison methods is illustrated when examining comparison method A. Inillustration, in transvenous studies, although a MVT arrhythmia willgenerally correlate poorly (score as LOW) when compared to a storedsinus template, there is an approximately 20 percent chance that a MVTmay demonstrate a high correlation and actually score HIGH using thesame comparison method. Thus, the influence of these more ambiguousresults is troubling when discriminating between arrhythmias, andultimately in directing therapy. Layered analysis may aid in reducingthe ambiguity. For example, layers having more than one method ofanalyzing data from a single sensing view, and layers analyzing datafrom more than one sensing view, may aid in reducing ambiguities.

The operational circuitry used in the implantable medical devices of thepresent invention may be configured to include such controllers,microcontrollers, logic devices, memory, and the like, as selected,needed, or desired for performing the steps for which each isconfigured.

In addition to uses in an ICD system, the present invention is alsoapplicable to pacing systems. For example, in a pacing system a numberof electrodes may be disposed to define several sensing vectors, and thepresent invention may guide the selection of and periodic updating ofsensing vectors.

In one illustrative example, the present invention is embodied in animplantable cardiac treatment system comprising an implantable canisterhousing operational circuitry and a plurality of electrodes electricallycoupled to the operational circuitry wherein the operational circuitryis configured and coupled to the electrodes to define at least a firstimplanted electrode pair and a second implanted electrode pair. Theoperational circuitry may be configured to perform the steps ofcapturing a first signal from the first implanted electrode pair,constructing a first template using the first signal, capturing a secondsignal from the second implanted electrode pair, constructing a secondtemplate using the second signal, and capturing a signal using the firstand second electrode pairs and using the first and second templates todetermine whether a treatable cardiac condition exists.

Numerous characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many aspects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size and arrangement of parts without exceeding the scope of theinvention. The invention's scope is defined, of course, in the languagein which the claims are expressed.

1. A method of cardiac signal analysis comprising: capturing a cardiacevent along first and second sensing vectors using electrodes implantedin a patient to yield first and second event representations; analyzingthe cardiac event by use of the first event representation to reach afirst result; observing whether the first result is determinative and,if not, analyzing the cardiac event by use of the second eventrepresentation.
 2. The method of claim 1, wherein the step ofdetermining whether the first result is determinative is performed in amanner wherein, if the first result unambiguously indicates anon-malignant cardiac rhythm, the first result is found to bedeterminative.
 3. The method of claim 1, wherein the step of analyzingthe cardiac event by use of the first event representation includesobserving a correlation between the first event representation and anevent template, wherein the first result is related to the correlation.4. The method of claim 3, wherein the first result is a score fromcorrelation waveform analysis performed between the first eventrepresentation and the cardiac template.
 5. The method of claim 3,wherein the first result is related to a variability in a set ofcorrelation waveform analysis scores, the set of correlation waveformanalysis scores including a correlation waveform analysis score for thefirst event representation and the event template, and a numbercorrelation waveform analysis scores for first event representations forprevious cardiac events and corresponding event templates.
 6. The methodof claim 1, wherein the step of capturing the cardiac event includes:observing the cardiac event with a first electrode set defining thefirst sensing vector, the first electrode set being disposed in aposition for far-field sensing of the cardiac event; and observing thecardiac event with a second electrode set defining the second sensingvector, the second electrode set being disposed in a position forfar-field sensing of the cardiac event.
 7. The method of claim 1,wherein the step of capturing the cardiac event includes: observing thecardiac event with a first electrode set defining the first sensingvector, the first electrode set being disposed subcutaneously inside thepatient; and observing the cardiac event with a second electrode setdefining the second sensing vector, the second electrode set beingdisposed subcutaneously inside the patient.
 8. The method of claim 1,wherein the step of capturing the cardiac event includes: observing thecardiac event with a first electrode set defining the first sensingvector, the first electrode set being disposed outside the patient'sthoracic cavity; and observing the cardiac event with a second electrodeset defining the second sensing vector, the second electrode set beingdisposed outside the patient's thoracic cavity.
 9. A method of cardiacsignal analysis comprising performing the following steps for a numberof cardiac events: capturing an individual cardiac event along first andsecond sensing vectors using electrodes implanted in a patient to yieldfirst and second event representations; analyzing the first and secondevent representations to generate a first metric corresponding to thefirst event representation and a second metric corresponding to thesecond event representation; comparing the first metric to the secondmetric; and selecting one of the first or second event representationsas a primary event representation, and identifying the non-selectedevent representation as an alternate event representation for theindividual cardiac event.
 10. The method of claim 9, further comprising:analyzing the primary event representation to generate a first result;observing whether the first result is determinative and, if not,analyzing the cardiac event by use of the alternate eventrepresentation.
 11. The method of claim 10, wherein the step ofdetermining whether the first result is determinative is performed in amanner wherein, if the first result unambiguously indicates anon-malignant cardiac rhythm, the first result is found to bedeterminative.
 12. The method of claim 10, wherein the step of analyzingthe primary event representation includes observing a correlationbetween the primary event representation and an event template, whereinthe first result is related to the correlation.
 13. The method of claim12, wherein the first result is a score from correlation waveformanalysis performed between the primary event representation and thecardiac template.
 14. The method of claim 9, wherein the step ofcapturing the cardiac event includes: observing the cardiac event with afirst electrode set defining the first sensing vector, the firstelectrode set being disposed in a position for far-field sensing of thecardiac event; and observing the cardiac event with a second electrodeset defining the second sensing vector, the second electrode set beingdisposed in a position for far-field sensing of the cardiac event. 15.The method of claim 9, wherein the step of capturing the cardiac eventincludes: observing the cardiac event with a first electrode setdefining the first sensing vector, the first electrode set beingdisposed subcutaneously inside the patient; and observing the cardiacevent with a second electrode set defining the second sensing vector,the second electrode set being disposed subcutaneously inside thepatient.
 16. The method of claim 9, wherein the step of capturing thecardiac event includes: observing the cardiac event with a firstelectrode set defining the first sensing vector, the first electrode setbeing disposed outside the patient's thoracic cavity; and observing thecardiac event with a second electrode set defining the second sensingvector, the second electrode set being disposed outside the patient'sthoracic cavity.
 17. A method of cardiac signal analysis comprising: A)capturing a cardiac event along first and second sensing vectors fromsensors implanted in a patient to yield first and second eventrepresentations; B) analyzing the cardiac event by use of the firstevent representation to reach a first result; C) observing whether thefirst result is determinative and, if not, analyzing the cardiac eventby use of the second event representation; wherein: the step ofdetermining whether the first result is determinative is performed in amanner wherein, if the first result unambiguously indicates anon-malignant cardiac rhythm, the first result is found to bedeterminative; the step of analyzing the cardiac event by use of thefirst event representation includes observing a correlation between thefirst event representation and an event template, wherein the firstresult is related to the correlation; and the step of capturing thecardiac event includes observing the cardiac event with a firstelectrode set defining the first sensing vector, the first electrode setbeing disposed in a position for far-field sensing of the cardiac event,and observing the cardiac event with a second electrode set defining thesecond sensing vector, the second electrode set being disposed in aposition for far-field sensing of the cardiac event.
 18. The method ofclaim 17, wherein steps A, B, and C are performed for a plurality ofcardiac events to determine whether the patient is experiencing amalignant cardiac condition.
 19. The method of claim 17, wherein eachelectrode making up part of the first electrode set and the secondelectrode set is disposed subcutaneously in the patient.
 20. The methodof claim 17, wherein each electrode making up part of the firstelectrode set and the second electrode set is disposed at a nonvascularlocation exclusive of the patients heart.